diff --git a/src/index.ts b/src/index.ts
index b00ec59..7ead61f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,5 +1,6 @@
 export function fibonacci(num: number): number | null {
   if (num < 0) {
+    console.log(`Error: the number ${num} is a negative value`);
     return null;
   }
 
@@ -13,6 +14,6 @@ export function fibonacci(num: number): number | null {
     num--;
   }
 
+  console.log(`return value is ${b}`);
   return b;
 }
-console.log("Hello");
\ No newline at end of file
diff --git a/tests/index.test.ts b/tests/index.test.ts
index 41f4e3b..642df47 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -1,5 +1,23 @@
-import { fibonacci } from "../src/index"
+import { fibonacci } from "../src/index";
 
-it("should return '1' for 1", () => {
-  expect(fibonacci(1)).toBe(1);
-})
+describe("fibonacci ", () => {
+  const consoleLogSpy = jest.spyOn(console, "log");
+
+  afterEach(() => {
+    jest.resetAllMocks();
+  });
+
+  it("should return '1' for 1", () => {
+    expect(fibonacci(1)).toBe(1);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith("return value is 1");
+  });
+
+  it("should return null for -1", () => {
+    expect(fibonacci(-1)).toBe(null);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith(
+      "Error: the number -1 is a negative value"
+    );
+  });
+});